The present invention relates to a vacuum microdevice and, more particularly, to the structure of a field emission cold cathode applied to, e.g., a microminiature microwave vacuum tube and a microminiature display element, and a method of manufacturing the same.
A microminiature field emission cold cathode can be manufactured as a vacuum microdevice by using silicon semiconductor technologies, and several conventional methods are known. To enhance the function of a field emission cold cathode, however, it is necessary to meet material requirements such as the use of an emitter material which has a small work function and hardly changes due to an environment, in addition to satisfying dimensional requirements such as a sharp tip of an emitter and the uniformity of shapes of a plurality of emitters. For this reason, a manufacturing method using the principle of a mold method has attracted attention recently. In this method, a recessed portion with a pointed bottom surface is formed in a silicon substrate, an emitter material is buried in the recessed portion, and the emitter is separated from the silicon substrate. A method of manufacturing a field emission cold cathode using this mold method was first reported in H. F. Grey et al., "Method of Manufacturing a Field-Emission Cathode Structure" (U.S. Pat. No. 4,307,507).
In this mold method, a large number of very small recessed portions can be uniformly formed in a silicon substrate, and processing is facilitated because it is only necessary to bury an emitter material in these recessed portions. Therefore, the method has the advantage that various types of emitter materials can be used. However, the patent to H. F. Grey et al. has the limitation that the thickness of an emitter must be increased since, if the emitter material is a thin film, the strength of the emitter is insufficient when the emitter is separated from the silicon substrate. This prolongs the emitter formation time, and a technique for controlling large stresses remaining in the emitter material is also necessary.
One method capable of manufacturing a cold cathode device by using a thin emitter film is to reinforce the thin emitter film by adhering the film to a structural substrate having a sufficient strength. An example of the manufacture of a triode structure device using this method is described in M. Nakamoto et al., "Manufacture of Field Emission Cold Cathode, Field Emission Cold Cathode Using It, and Flat Image Display" (Japanese Patent Laid-Open No. 6-36682). This prior art will be described below with reference to FIGS. 9 and 10A to 10F. FIG. 9 shows the structure of a field emission cold cathode using the mold method. An emitter electrode 101 having sharp tips in current radiation regions 104 is formed on a glass substrate 100. A gate electrode 103 is formed on the emitter electrode 101 via an oxide film 102.
When a voltage of about 100 V is applied between the gate electrode 103 and the emitter electrode 101, an intense electric field of about 10.sup.9 V/cm is generated because the tip of the emitter electrode 101 is sharpened in the current radiation region 104. Electrons are emitted from the tip of the emitter electrode 101 due to this intense electric field. Since the current radiation region 104 thus generates an intense electric field, it is required to control the shape of the emitter electrodes and the gate electrodes 103 with high accuracy.
FIGS. 10A to 10F illustrate a method of manufacturing the structure shown in FIG. 9 in the order of steps. As shown in FIG. 10A, holes 116 each having dimensions of 1 .mu.m.times.1 .mu.m.times.0.7 .mu.m are formed in a silicon substrate 110 by using an oxide film 111 as a mask. In this formation, holes having the shape of an inverted triangular pyramid can be easily formed by etching the silicon substrate 110 by using a KOH (potassium hydroxide) solution. Subsequently, as shown in FIG. 10B, the silicon substrate 110 is oxidized to form an oxide film 112 about 300 nm thick inside the holes 116. An emitter metal 113 is deposited to have a thickness of about 1 .mu.m on the oxide film 112. Forming the oxide film 112 in the holes 116 has an effect of sharpening the points of the holes 116. As shown in FIG. 10C, the emitter metal 113 and a glass substrate 100 are adhered by using electrostatic adhesion. The resultant sample is then dipped in a KOH etching solution to completely remove the silicon substrate 110. Since a KOH etching solution has a silicon etching rate approximately 100 times as high as that of an oxide film, the structure shown in FIG. 10C is obtained.
Subsequently, as shown in FIG. 10D, a resist 115 is applied on the surface of a gate metal 114 about 1 .mu.m thick formed by sputtering. Molybdenum is commonly used as the emitter metal 113 and the gate metal 114. As shown in FIG. 10E, the resist 115 is back-etched under conditions by which the entire surface of the sample is etched at a uniform rate. The back-etch is stopped when the oxide film 112 is exposed in regions 117 where sharp tips are formed. Thereafter, as shown in FIG. 10F, the resist 115 is removed, and the sample is dipped in an HF (hydrogen fluoride) solution to etch the oxide film 112 exposed in the regions 117. Consequently, the tips of the metal 113 as the emitter electrode can be exposed.